Transcript Chapter 19 - The Preprocessor

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Chapter 19 - The Preprocessor
Outline
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
19.9
19.10
Introduction
The #include Preprocessor Directive
The #define Preprocessor Directive: Symbolic Constants
The #define Preprocessor Directive: Macros
Conditional Compilation
The #error and #pragma Preprocessor Directives
The # and ## Operators
Line Numbers
Predefined Symbolic Constants
Assertions
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19.1 Introduction
• Preprocessing
– Occurs before program compiled
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Inclusion of external files
Definition of symbolic constants
Macros
Conditional compilation
Conditional execution
– All directives begin with #
• Can only have whitespace before directives
– Directives not C++ statements
• Do not end with ;
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19.2 The #include Preprocessor Directive
• #include directive
– Puts copy of file in place of directive
• Seen many times in example code
– Two forms
• #include <filename>
– For standard library header files
– Searches predesignated directories
• #include "filename"
– Searches in current directory
– Normally used for programmer-defined files
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19.2 The #include Preprocessor Directive
• Usage
– Loading header files
• #include <iostream>
– Programs with multiple source files
– Header file
• Has common declarations and definitions
• Classes, structures, enumerations, function prototypes
• Extract commonality of multiple program files
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19.3 The #define Preprocessor Directive:
Symbolic Constants
• #define
– Symbolic constants
• Constants represented as symbols
• When program compiled, all occurrences replaced
– Format
• #define identifier replacement-text
• #define PI 3.14159
– Everything to right of identifier replaces text
• #define PI=3.14159
• Replaces PI with "=3.14159"
• Probably an error
– Cannot redefine symbolic constants
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19.3 The #define Preprocessor Directive:
Symbolic Constants
• Advantages
– Takes no memory
• Disadvantages
– Name not be seen by debugger (only replacement text)
– Do not have specific data type
• const variables preferred
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19.4 The #define Preprocessor Directive:
Macros
• Macro
– Operation specified in #define
– Intended for legacy C programs
– Macro without arguments
• Treated like a symbolic constant
– Macro with arguments
• Arguments substituted for replacement text
• Macro expanded
– Performs a text substitution
• No data type checking
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19.4 The #define Preprocessor Directive:
Macros
• Example
#define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) )
area = CIRCLE_AREA( 4 );
becomes
area = ( 3.14159 * ( 4 ) * ( 4 ) );
• Use parentheses
– Without them,
#define CIRCLE_AREA( x ) PI * x * x
area = CIRCLE_AREA( c + 2 );
becomes
area = 3.14159 * c + 2 * c + 2;
which evaluates incorrectly
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19.4 The #define Preprocessor Directive:
Macros
• Multiple arguments
#define RECTANGLE_AREA( x, y ) ( ( x ) * ( y ) )
rectArea = RECTANGLE_AREA( a + 4, b + 7 );
becomes
rectArea = ( ( a + 4 ) * ( b + 7 ) );
• #undef
– Undefines symbolic constant or macro
– Can later be redefined
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19.5 Conditional Compilation
• Control preprocessor directives and compilation
– Cannot evaluate cast expressions, sizeof, enumeration
constants
• Structure similar to if
#if !defined( NULL )
#define NULL 0
#endif
– Determines if symbolic constant NULL defined
– If NULL defined,
• defined( NULL ) evaluates to 1
• #define statement skipped
– Otherwise
• #define statement used
– Every #if ends with #endif
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19.5 Conditional Compilation
• Can use else
– #else
– #elif is "else if"
• Abbreviations
– #ifdef short for
• #if defined(name)
– #ifndef short for
• #if !defined(name)
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19.5 Conditional Compilation
• "Comment out" code
– Cannot use /* ... */ with C-style comments
• Cannot nest /* */
– Instead, use
#if 0
code commented out
#endif
– To enable code, change 0 to 1
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19.5 Conditional Compilation
• Debugging
#define DEBUG 1
#ifdef DEBUG
cerr << "Variable x = " << x << endl;
#endif
– Defining DEBUG enables code
– After code corrected
• Remove #define statement
• Debugging statements are now ignored
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19.6 The #error and #pragma Preprocessor
Directives
• #error tokens
– Prints implementation-dependent message
– Tokens are groups of characters separated by spaces
• #error 1 - Out of range error has 6 tokens
– Compilation may stop (depends on compiler)
• #pragma tokens
– Actions depend on compiler
– May use compiler-specific options
– Unrecognized #pragmas are ignored
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19.7 The # and ## Operators
• # operator
– Replacement text token converted to string with quotes
#define HELLO( x ) cout << "Hello, " #x << endl;
– HELLO( JOHN ) becomes
• cout << "Hello, " "John" << endl;
• Same as cout << "Hello, John" << endl;
• ## operator
– Concatenates two tokens
#define TOKENCONCAT( x, y )
– TOKENCONCAT( O, K ) becomes
• OK
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x ## y
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19.8 Line Numbers
• #line
– Renumbers subsequent code lines, starting with integer
• #line 100
– File name can be included
– #line 100 "file1.cpp"
• Next source code line is numbered 100
• For error purposes, file name is "file1.cpp"
• Can make syntax errors more meaningful
• Line numbers do not appear in source file
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19.9 Predefined Symbolic Constants
• Five predefined symbolic constants
– Cannot be used in #define or #undef
Symbolic
constant
Description
__LINE__
The line number of the current source code line (an integer constant).
__FILE__
The presumed name of the source file (a string).
__DATE__
The date the source file is compiled (a string of the form "Mmm dd
yyyy" such as "Jan 19 2001").
__TIME__
The time the source file is compiled (a string literal of the form
"hh:mm:ss").
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19.10 Assertions
• assert is a macro
– Header <cassert>
– Tests value of an expression
• If 0 (false) prints error message, calls abort
– Terminates program, prints line number and file
– Good for checking for illegal values
• If 1 (true), program continues as normal
– assert( x <= 10 );
• To remove assert statements
– No need to delete them manually
– #define NDEBUG
• All subsequent assert statements ignored
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